Multiply structured particle and method for producing the same

ABSTRACT

In the present invention, when hollow polyhedral fine particles consisting of atoms of a first element and atoms of a second element are produced, atoms of the first element and atoms of the second element are structured in a reversed micelle composed of a surfactant. Thereby, hollow polyhedral fine particles can be synthesized by simple and easy procedures.

TECHNICAL FIELD

The present invention relates to a method for producing hollow fineparticles, and particularly relates to a method for producing hollowpolyhedral fine particles having a very small particle size on the orderof nanometers. In addition, the present invention also relates to anovel hollow polyhedral fine particle produced by the aforementionedpreparation method.

BACKGROUND ART

Hollow polyhedral nonsocial particles consisting of from several atomsto several thousand atoms exhibit physical properties which aredifferent from those of bulk crystal structures consisting of the sameatoms, and the physical properties of the nonsocial particles greatlyvary in accordance with the number of structural atoms, the atomicarrangement states, and the like. For this reason, research thereon hasbeen carried out from a broad perspective of them as “new materials”applicable to various uses.

As the hollow polyhedral particles described above, carbon fullerenessuch as Chad 60 and the like are already known. However, hollowpolyhedral fine particles which are stable in the atmosphere and arenext to the carbon fullerenes in rank have not been discovered orcreated. In addition, the carbon fullerenes are synthesized by highenergy consuming methods such as vapor growth methods, arching methods,and the like. Therefore, production costs of the carbon fullerenes areextremely high due to low production efficiency, and therefore,practical application thereof to various uses is inhibited.

DISCLOSURE OF THE INVENTION

A main object of the present invention is to synthesize hollowpolyhedral fine particles in large amounts through simple organ chemicalsynthetic methods without using a large amount of energy.

In addition, another object of the present invention is to synthesizenovel hollow polyhedral fine particles by the aforementioned organchemical synthetic methods.

The objects of the present invention can be achieved by synthesizinghollow polyhedral fine particles in which, when hollow polyhedral fineparticles consisting of atoms of a first element and atoms of a secondelement are produced, the aforementioned first element atoms and theaforementioned second element atoms are structured in a reversed micellecomposed of a surfactant.

In particular, when hollow polyhedral fine particles consisting of atomsof the first element and atoms of the second element are produced, inthe present invention, the desirable hollow polyhedral fine particlescan be easily produced by essentially carrying out the steps describedbelow:

a first step of dissolving or dispersing a surfactant, a compoundcontaining atoms of the aforementioned first element, and a compoundcontaining atoms of the aforementioned second element, in an aqueousmedium to obtain an aqueous solution or an aqueous dispersion;

a second step of adding an oily medium to the aforementioned aqueoussolution or aqueous dispersion to obtain a double phase contactingliquid in which an aqueous phase and an oily phase directly contact;

a third step of forming a reversed micelle composed of theaforementioned surfactant in the aforementioned oily phase of theaforementioned double phase contacting liquid; and

a fourth step of structuring the aforementioned first element atoms andthe aforementioned second element atoms in the aforementioned reversedmicelle to obtain hollow polyhedral fine particles.

In the method for producing hollow polyhedral fine particles of thepresent invention, after the aforementioned fourth step, a fifth step ofseparating and recovering the aforementioned hollow polyhedral fineparticles from the aforementioned oily phase is preferably carried out.

In the method for producing hollow polyhedral fine particles of thepresent invention, the aforementioned first element may be the same asthe aforementioned second element, or alternatively may be differentfrom the aforementioned second element.

In the case in which the aforementioned first element is different fromthe aforementioned second element, it is preferable that theaforementioned first element be Cod, and the aforementioned secondelement be Se. Thereby, novel hollow polyhedral fine particles having achemical formula of (CdSe)₃₃ or (CdSe)₃₄ can be produced in largeamounts with ease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical absorption spectral chart of a product obtained inExample 1.

FIG. 2 is a mass spectral chart of the product obtained in Example 1.

FIG. 3 is an electron microscope photo of the product obtained inExample 1.

FIG. 4 is a first drawing showing a stable structure of (CdSe)₃₃ and(CdSe)₃₄.

FIG. 5 is a second drawing showing a stable structure of (CdSe)₃₃ and(CdSe)₃₄.

FIG. 6 is an X-ray diffraction spectral chart of the product obtained inExample 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The method for producing the hollow polyhedral fine particles of thepresent invention is based on a technical idea in which hollowpolyhedral fine particles consisting of a first element and a secondelement are produced by structuring the first element and the secondelement in a namespace in a reversed micelle formed by a surfactant. Inthe following, specific steps of carrying out the production method ofthe present invention are described in detail after each of the elementsinvolved in the production method of the present invention is defined.

Definition of Terms:

(1) Hollow Polyhedral Fine Particles

“Hollow polyhedral fine particles” when employed in the presentinvention means individual very fine particles having a hollow cavitypart within the particle. In the aforementioned particle, theaforementioned hollow cavity part is separated from the external worldby a very thin shell having a polyhedron structure. The aforementionedpolyhedron structure is not necessarily a regular polyhedron, and thesurface thereof may be formed by various polygons such as triangles,quadrangles, pentagonal shapes, and the like.

The hollow polyhedral fine particles of the present invention have asize in which the distance between two atoms forming the aforementionedfine particle, which are the farthermost from each other, ranges from0.1 to 20 nm, preferably ranges from 0.5 to 10 nm, more preferablyranges from 0.8 to 5.0 nm, and in particular, preferably ranges from 1.0to 3.0 nm. The shapes of the hollow polyhedral fine particles are notparticularly limited, and may be various shapes such as regularhexahedrons, cuboids, cylindrical bodies, plate-shaped bodies, generalspherical bodies, and the like. When the hollow polyhedral fineparticles are in the form of a general spherical body, the particle sizeof the aforementioned spheres corresponds to the size described above.

In addition, the inner spaces in the hollow polyhedral fine particles ofthe present invention preferably have a volume which can contain one ormore atoms. By containing at least one atom in the aforementioned innerspace, physical properties of the fine particles per se can greatlyvary, compared with the case of containing no atoms in the inner space.

The hollow polyhedral fine particles of the present invention aredifferent from a cluster of a bulk crystal composed of the atoms of oneor more elements which are the same as those forming the aforementionedfine particles, and the inner spaces of the hollow polyhedral fineparticles are not filled with atoms of the elements described above.That is, the hollow polyhedral fine particles of the present inventiononly have a shell structure formed from atoms of the elementsconstituting the aforementioned fine particles, and the inside thereofis completely hollow. It is not necessary to completely shield the innerspace of the hollow polyhedral fine particle from the outside, and apart of the aforementioned inner space may be in communication with theouter world, but it is preferable that the inner space be completelyshielded from the outer world.

(2) Structuring

“Structuring” when employed in the present invention means constructinga three-dimensional structure by accumulating the constituent elements.In particular, this means that a specific unit of atoms of the firstelement and atoms of the second element is accumulated to constitute ashell structure, thereby finally forming a hollow polyhedral fineparticle.

The “unit” of atoms of the first element and atoms of the second elementemployed herein may be composed of atoms of the first element alone,atoms of the second element alone, or a combination of atoms of thefirst element and atoms of the second element. When the first element isdifferent from the second element, a combination of the atoms of thefirst and second elements is preferably employed, and in particular, acombination of one first element atom and one second element atom ispreferable in view of geometric properties of the shell structure. Forexample, when Cod is employed as the first element atom, and Se isemployed as the second element atom, a hollow polyhedral fine particlecan be constituted by structuring a combination of CdSe as a “unit”.

(3) Surfactant

“Surfactant” when employed in the present invention means a well-knownsubstance exhibiting surface active effects, and in particular, means aconventional surfactant in which a hydrophilic group and a hydrophobicgroup are present in one molecule. In general, a surfactant forms amicelle in an aqueous phase, or alternatively forms a reversed micellein an oily phase, depending on the ratio of the molecular weight of thehydrophilic group and the molecular weight of the hydrophobic group inthe molecule of the aforementioned surfactant. As the surfactantemployed in the production method of the present invention, one whichcan form a reversed micelle is employed.

Specific Steps For Carrying Out The Method of the Present Invention:

(1) First Step

In the method for producing hollow polyhedral fine particles of thepresent invention, first, a surfactant, a compound containing atoms ofthe aforementioned first element, and a compound containing atoms of theaforementioned second element are dissolved or dispersed in an aqueousmedium, and thereby an aqueous solution or an aqueous dispersion isobtained.

The kind of aqueous medium employed in the first step can be determineddepending on the kinds of the first element, the second element, and/orthe surfactant. As the aqueous medium, water; lower alcohols such asmethanol, ethanol, propel alcohol, isopropyl alcohol, tart-butylalcohol, and the like; ketenes such as acetone, methyl ethyl ketene, andthe like; ethers such as diethyl ether, methyl ethyl ether, diethylether, and the like; and the like, can be employed, but the aqueousmedium is not limited thereto. As the aqueous medium, water or alcoholsare preferable. In particular, alcohols exhibit effects of acceleratingformation of a reversed micelle composed of the surfactant in the thirdstep described below. For this reason, a mixed medium of water and analcohol is particularly preferable as the aqueous medium.

As the first element, the group II to VI elements are preferable, andthe group II elements which can form a divalent action, such as Cod, Zn,and the like, are preferable. In particular, Cod is preferable. However,the first element is not limited thereto. As the second element, whichis not particularly limited, the group II to VI elements are preferable,and the group VI elements which can form a divalent anion, such as S,Se, Te, and the like, are preferable. In particular, Se is preferable.Therefore, when the first element is different from the second element,in particular, a combination of Cod and Se is preferable. When the firstelement and the second element are the same, the group IV elements suchas C, Si, Gee, and the like, which can form the different apparentoxidation values, are preferable, and in particular, Si is preferable.

The form of the compound containing atoms of the first element is notparticularly limited. In view of solubility in an aqueous medium,compounds which can supply atoms of the first element as ions, andparticularly captions, are preferable. For example, salts containingatoms of the first element as captions are preferable. For example, inthe case of employing Cod as atoms of the first element, CdSO₄ or thelike can be employed. CdSO₄ has a high solubility in water (76.7 g per100 g of water), and for this reason, it is preferable. Similarly, theforms of the compounds containing atoms of the second element are notparticularly limited. In view of solubility in an aqueous medium,compounds which can supply atoms of the second element as ions, andparticularly anions, are preferable. For example, salts containing atomsof the second element as anions are preferable. For example, in the caseof employing Se as atoms of the second element, Na₂SeSO₃ or the like canbe employed.

When a compound such as a salt supplying the first element atoms ascations is employed as the compound containing atoms of the firstelement, and a compound such as a salt supplying the second elementatoms as anions is employed as the compound containing atoms of thesecond element, large particles may be produced by reacting the firstelement atoms and the second element atoms immediately after mixing boththe element atoms in some cases. For this reason, in order to preventthe aforementioned reaction, it is preferable that a chelating agent bepresent together therewith. The chelating agent encompasses the firstelement atoms and/or the second element atoms to form a cyclic structureand stabilize these. Thereby, the ratio of the first element atomsand/or the second element atoms both which are free in the reactionsystem is extremely limited, and the reaction between the first elementatoms and the second element atoms is delayed, and thereby, formation oflarge particles can be controlled.

For example, in the case of employing Cd as the first element atoms, itis preferable that SNTA (N(CH₂COONa)₃) be present as a chelating agenttogether therewith. Thereby, as conceptualized in the following, theCd²⁺ ion present in the aqueous medium is stabilized by forming a cyclicstructure due to coordinate bonds with the N atom and the O atom ofSNTA. For this reason, the amount of the free Cd²⁺ in the aqueous mediumis extremely reduced, and the reaction rate of Cd2+ in the aqueousmedium can be extremely reduced.

As surfactants, various surfactants such as nonionic, cationic, anionic,and amphoteric surfactants can be employed as long as they can form areversed micelle in an oily phase. As examples of the surfactants,mention may be made of, for example, sulfate type anionic surfactantssuch as salts of C₁₂-C₁₈ saturated or unsaturated alkyl sulfuric acids,salts of C₁₂-C₁₈ saturated or unsaturated alkyl ether sulfuric acids,salts of C₁₂-C₁₈ saturated or unsaturated alkyl aryl ether sulfuricacids, and the like; sulfonate type anionic surfactants such as salts ofC₁₂-C₂₂ saturated or unsaturated alkyl sulfonic acids, salts of C₈-C₂₂saturated or unsaturated alkylbenzene sulfonic acids, salts of α-olefinsulfonic acids, and the like; carboxylic acid type anionic surfactantssuch as salts of C₁₂-C₁₈ saturated or unsaturated fatty acids, salts ofC₁₂-C₁₈ saturated or unsaturated alkyl ether carboxylic acids, salts ofN-acylglutamic acid, and the like; phosphate type anionic surfactantssuch as salts of C₁₂-C₁₈ saturated or unsaturated alkyl phosphoricacids, salts of POE mono- or di(C₈-C₁₂ saturated or unsaturated alkyl)ether phosphoric acids, salts of C₈-C₁₂ saturated or unsaturated alkylaryl ether phosphoric acids, and the like; alkylamine type anionicsurfactants such as (C₈-C₁₂ saturated or unsaturated alkyl)amines, andthe like; ammonium type cationic surfactants such as C₁₂-C₁₈ saturatedor unsaturated alkylammonium chloride, C₁₂-C₁₈ saturated or unsaturatedalkyl ether ammonium chloride, and the like; benzalkonium type cationicsurfactants such as C₁₂-C₁₈ saturated or unsaturated alkyl dimethylbenzalkonium chloride, octylphenoxy ethoxyethyl dimethylbenzylammoniumchloride, and the like; betaine type amphoteric surfactants such asdi(C₈-C₁₂ saturated or unsaturated alkyl) diaminoethyl betaines, C₁₂-C₁₈saturated or unsaturated alkyl dimethylbenzyl betaines, and the like;glycine type amphoteric surfactants such as C₁₂-C₁₈ saturated orunsaturated alkyl dimethylbenzyl glycine, and the like;polyoxyethylene/polyoxypropylene block polymer type nonionic surfactantssuch as polyoxyethylene/polyoxypropylene block polymers, C₁₂-C₁₈saturated or unsaturated alkyl/polyoxyethylene/polyoxypropylene blockpolymer ethers, and the like; sugar ester type nonionic surfactants suchas sorbitan C₁₂-C₁₈ fatty acid esters, POE-sorbitan C₁₂-C₁₈ fatty acidesters, and the like; fatty acid ester type nonionic surfactants such asPOE-C₁₂-C₁₈ fatty acid esters and the like; POE ether type nonionicsurfactants such as POE-(C₈-C₁₂ saturated or unsaturated alkyl) ethers,POE-(C₈-C₁₂ saturated or unsaturated alkyl) phenyl ethers, and the like;silicone type nonionic surfactants such as POE-modified silicones, andthe like, which are known surfactants. One kind of surfactant may beemployed or surfactants may be employed in combination of two or morekinds thereof.

The surfactants employed in the present invention are preferably thosewhich can be easily dissolved or dispersed in the aqueous medium. Inaddition, in the second step described below, the surfactants employedin the present invention are preferably those which do not inhibitdirect contact between the aqueous phase and the oily phase, and caneasily form a large amount of reversed micelles in the oily phase. Asexamples of surfactants having the preferable properties describedabove, mention may be made of, for example, one-dimensional anionicsurfactants having C₈-C₂₂ long-chain saturated alkyl groups, and inparticular, long-chain saturated alkyl amines having NH₂— groups ashydrophilic groups, and octyl (CH₃(CH₂)₇) groups, decyl (CH₃(CH₂)₉)groups, dodecyl (CH₃(CH₂)₁₁) groups, or the like, as hydrophobic groups.

In addition, the size of the space in the reversed micelle which thesurfactant forms in the second step described below tends to depend onthe size of the molecule of the surfactant, per se. For this reason, byselecting a surfactant having an appropriate molecular length, thediameter of the nanospace for the reaction in the reversed micelle canbe controlled. For example, if the bond lengths of C—C, C—H, C—N, andN—H are respectively 1.54 angstroms, 1.10 angstroms, 1.47 angstroms, and1.00 angstroms, the molecular length of the decylamine (CH₃(CH₂)₉NH₂) isapproximately 1.8 nm, and for this reason, the nanospace in the reversedmicelle can be a spherical space having a diameter of not more thanapproximately 2 nm. As described above, by properly selecting thesurfactants, the size of the hollow polyhedral fine particles finallyproduced can be controlled.

Dissolving or dispersing the compound containing the first elementatoms, the compound containing the second element atoms, and thesurfactant in the aqueous medium can be carried out by means of a knownstirring and mixing apparatus such as a contacting type of mixing andstirring apparatus with a stirrer such as a magnetic stirrer or thelike, a rotor-stator type mixing and stirring apparatus such as acolloid mill, a homogenizer, or the like, or a non-contacting typemixing and stirring apparatus employing ultrasonic waves or the like.

The specific methods for adding the compound containing the firstelement atoms, the compound containing the second element atoms, and thesurfactant to the aqueous medium are not particularly limited. They maybe respectively added to the aqueous medium as they are, oralternatively, they may be formed into a small amount of an aqueoussolution or an aqueous dispersion, followed by adding to theaforementioned aqueous medium. In the case of previously producing asmall amount of the aqueous solution or the aqueous dispersion whichincludes the compound containing the first element atoms and/or thecompound containing the second element atoms, in order to control thestates of the first element atoms and the second element atoms in theaforementioned solution or dispersion, it is preferable that the pH ofthe aforementioned aqueous solution or aqueous dispersion be previouslyset in a specific range.

The order of adding the compound containing the first element atoms, thecompound containing the second element atoms, and the surfactant to theaqueous medium is not particularly limited. For example, a mixture ofthe compound containing the first element atoms (or the aqueous solutionor aqueous dispersion thereof) and the surfactant is added to theaqueous medium, and subsequently, the compound containing the secondelement atoms (or the aqueous solution or aqueous dispersion thereof)can be added thereto.

The temperature of the aqueous phase obtained by dissolving ordispersing the compound containing the first element atoms, the compoundcontaining the second element atoms, and the surfactant in the aqueousmedium is preferably controlled to be in a specific range. In the caseof extremely high temperatures, the reaction rate of the first elementatoms and the second element atoms is increased, and a large particlemay be produced. On the other hand, in the case of extremely lowtemperatures, the rate of dissolving or dispersing the compoundcontaining the first element atoms, the compound containing the secondelement atoms and the surfactant in the aqueous medium is decreased inthe aqueous medium. Therefore, the temperature of the aqueous phase ispreferably controlled to be in an appropriate range. The specifictemperature range depends on the kinds of the first element, the secondelement, and the surfactant, and depends on other factors. Thetemperature is controlled typically to be in the range of 15° C. to 80°C., more typically in the range of 20° C. to 75° C., and in particular,typically in the range of 25° C. to 75° C.

When the aforementioned aqueous phase contains a chelating agent, the pHof the aforementioned aqueous phase is preferably controlled to be in anappropriate range. The specific pH range depends on the kinds of thefirst element, the second element, and the chelating agent, and dependson other factors. For example, in the case of employing Cd as the firstelement and employing SNTA as the chelating agent, in order to securestability of the chelate, the pH is controlled typically to be in therange of from 8 to 12, and more typically in the range of from 9 to 11.More particularly, the pH of the aqueous phase can be adjusted by addingan acid such as hydrochloric acid, sulfuric acid, nitric acid, formicacid, acetic acid, citric acid, or the like, or a base such as sodiumhydroxide, potassium hydroxide, sodium carbonate, ammonia, or the like.

(2) Second Step

In the method for producing hollow polyhedral fine particles of thepresent invention, there is a second step in which an oily medium isadded to an aqueous phase obtained in the first step to obtain a doublephase contacting liquid in which the aqueous phase and the oily phasedirectly contact.

The specific kind of oily medium employed in the second step isdetermined depending on the kinds of the first element, the secondelement, and/or the surfactant. As the oily medium, vegetable oils suchas olive oil, castor oil, and the like; saturated hydrocarbons such aspentane, hexane, cyclopentane, cyclohexane, liquid paraffin, and thelike; aromatic hydrocarbons such as benzene, xylene, toluene, and thelike; esters such as isopropyl myristate, 2-octyldodecyl myristate, andthe like; higher alcohols such as isostearyl alcohols, and the like;higher fatty acids such as stearic acid, lauric acid, and the like;silicone oils such as dimethylpolysiloxane, methylphenylpolysiloxane,and the like; and the like, can be employed. However, the oily medium isnot limited thereto.

In the second step, it is necessary to obtain a condition in which theaqueous phase directly contacts the oily phase without via thesurfactant, after adding the oily medium to the aqueous phase. Due todirect contact between the aqueous phase and the oily phase, thesurfactant in the aqueous phase can migrate in the oily phase togetherwith the first element atoms and the second element atoms.

The addition of the oily medium to the aqueous phase is preferablycarried out immediately after formation of the aqueous phase. Soon afterforming the aqueous phase by dissolving or dispersing the compoundcontaining the first element atoms, the compound containing the secondelement atoms, and the surfactant in the aqueous medium, the reactionbetween the first element atoms and the second element atoms begins inthe aforementioned aqueous phase or this reason, by adding the oilymedium immediately after formation of the aqueous phase, excess chainreactions can be prevented, and the first and second element atoms canrapidly immigrate into the oily phase together with the surfactant.

The double phase contacting liquid in which the aqueous phase directlycontacts the oily phase is preferably under a condition in which thecontinuous aqueous phase and the continuous oily phase form layers aboveand below, and they directly contact each other. If necessary, thedouble phase contacting liquid may be mixed and stirred using a stirrersuch as a magnetic stirrer or the like or using ultrasonic waves.

The temperature of the double phase contacting liquid is preferably atemperature of not lower than the Kraft point of the surfactant. Byhaving the temperature of not lower than the Kraft point of thetemperature of the double phase contacting liquid, the formation of thereversed micelles in the oily phase in the third step described belowcan be accelerated. The specific temperature range of the double phasecontacting liquid depends on the kinds of the surfactants and depends onother factors, and is controlled typically to be in the range of 15° C.or higher, more typically in the range of 20° C. or higher, and inparticular, typically in the range of 25° C. or higher. Theconcentration of the surfactant in the oily phase of the double phasecontacting liquid at the Kraft point approximately equals the critical(reversed) micelle concentration.

(3) Third Step

In the method for producing hollow polyhedral fine particles of thepresent invention, next, a third step in which reversed micellescomposed of the surfactant are formed in the oily phase of the doublephase contacting liquid obtained in the second step is carried out.

The temperature of the oily phase during forming the reversed micellesis preferably not lower than the Kraft point of the surfactant, but ispreferably not higher than the temperature at which the formation of thereversed micelles is not inhibited. If the temperature is too high,frequency of collisions between the reversed micelles in the oily phaseis raised, and the reversed micelles are broken. Therefore, it is notpreferable. The specific temperature range of the oily phase duringforming the reversed micelles depends on the kinds of the surfactantsand depends on other factors. It is controlled typically to be in therange of from 15° C. to 80° C., more typically in the range of from 20°C. to 75° C., and in particular, typically in the range of from 25° C.to 75° C. For example, in the case of employing decylamine as thesurfactant, the temperature of the oily phase is controlled to be from25° C. to 60° C.

The reversed micelles are formed from the surfactant which immigratesfrom the aqueous phase into the oily phase, and the first element atomsand the second element atoms are incorporated into the nanospace of theaforementioned reversed micelles. The nanospaces in the reversedmicelles are the aqueous phase, and for this reason, both the firstelement atoms and the second element atoms are preferably in ahydrophilic state. In this case, the first element atoms and the secondelement atoms can be intensively incorporated into the aforementionednanospace. As examples of the hydrophilic states of the first elementatoms and the second element atoms, mention may be made of an ionicstate of each of the element atoms or a water-soluble combination of thefirst and second element atoms.

By heating the oily phase to not lower than the Kraft point of thesurfactant the formation of the reversed micelles is almostautomatically carried out. For this reason, other specific treatments inorder to form the reversed micelles are unnecessary. If necessary, theaqueous medium or the oily medium may be added to in order to obtain adesired range of the volume ratio of the aqueous phase or the oilyphase.

(4) Fourth Step

In the method for producing hollow polyhedral fine particles of thepresent invention, next, a fourth step in which the first element atomsand the second element atoms are structured in the reversed micellesformed in the third step to produce hollow polyhedral fine particles iscarried out.

It is believed that the first element atoms and the second element atomsincorporated into the nanospace in the reversed micelle are respectivelyand independently structured to form a shell structure of the hollowpolyhedral fine particle, or alternatively, a specific unit is formed bycombining a specific ratio of the first element atoms and the secondelement atoms, and the aforementioned unit is structured to form a shellstructure of the hollow polyhedral fine particle. The former or latterstructuring method described above may be carried out, depending on thekinds of the first element and the second element, and depending onother factors.

The shell structure formed in the nanospace in the reversed micellegradually grows, but finally, this is limited to the size of theaforementioned nanospace, and thereby, a hollow polyhedral fine particlehaving a very minute size is formed. The geometrical structures andcompounds of the hollow polyhedral fine particles depend on the kinds ofthe first element and the second element and depends on stability of theaforementioned geometrical structures or the like. For example, in thecase of employing Cd as the first element and employing Se as the secondelement, hollow polyhedral fine particles represented by the chemicalformulae (CdSe)₃₃ and (CdSe)₃₄ can be formed. This reveals that thehollow polyhedral fine particles composed of only Cd and Se arestabilized in specific numbers (magic numbers) of Cd and Se atoms. Asdescribed above, in the present invention, hollow polyhedral fineparticles having a stable structure depending on the kinds of theelements employed can be produced.

Typically, the fourth step is successively carried out after theaforementioned third step. The fourth step is continued until the hollowpolyhedral fine particles are completely produced in the reversedmicelles formed in the third step. The reaction is continued bycontrolling the temperature of the oily phase of the double phasecontacting liquid in the same manner as described in the third step,typically for 10 minutes to 2 hours, more typically for 10 minutes toone hour, and further more typically for 10 minutes to 30 minutes.

(5) Fifth Step

In the method for producing the hollow polyhedral fine particle of thepresent invention, after the fourth step, a fifth step in which thehollow polyhedral fine particles produced from the oily phase areseparated and recovered is preferably carried out.

Typically, the fifth step is carried out by separating the oily phasefrom the double phase contacting liquid by means of a separating funnelor the like, and further separating the aforementioned oily phase intosolid and the liquid by means of filtration, centrifuging, or the like,thus recovering the hollow polyhedral fine particles on which thesurfactant is adhered. Subsequently, if necessary, the surfactant isremoved from the hollow polyhedral fine particles, and the fineparticles are dried. Thus, the hollow polyhedral fine particles can beproduced.

By intentionally leaving the surfactant on the surface of the hollowpolyhedral fine particles, a form in which chemical modification due tothe surfactant is performed on the surface of the hollow polyhedral fineparticle can be obtained. As described above, by varying the surfaceproperties of the hollow polyhedral fine particles, the hollowpolyhedral fine particles can be regularly arranged. For example, in thecase of employing Cd as the first element, employing Se as the secondelement, and employing decylamine as the surfactant, particles of(CdSe)₃₃ and (CdSe)₃₄ can be regularly arranged via the decylaminemolecules.

EXAMPLES

In the following, the present invention is described in detail by way ofExamples. It should be understood that the present invention is notlimited to these Examples.

Synthesis of Hollow Polyhedral Fine Particles of (CdSe)₃₃ and (CdSe)₃₄:

In the present Example, hollow polyhedral fine particles were producedemploying Cd as the first element and employing Se as the secondelement. The raw materials employed in the present Example are describedin the following.

-   Compound containing atoms of the first element: CdSO₄-   Compound containing atoms of the second element: Na₂SeSO₃-   Surfactant: saturated long-chain alkylamine

Example 1

(1) Preparation of Solution A

CdSO₄, in an amount of 2.75 mmol, was dissolved in 7 ml of water, and3.5 mmol of trisodium nitrilotriacetate (STNA: N(CH₂COONa)₃) as achelating agent was further dissolved therein. The mixture was stirredfor 5 minutes at room temperature, thus producing Solution A.

As conceptually shown in the scheme described below, SNTA and the freeCd²⁺ in the reaction system form a chelate in Solution A. For thisreason, the concentration of Cd²⁺ in the aqueous medium is very small.CdSO₄→Cd²⁺+SO₄ ²⁻Cd²⁺+NTA³⁻⇄Cd(NTA)⁻+NTA³⁻⇄Cd(NTA)⁴⁻

In addition, due to the reduced concentration of Cd²⁺, production ofCd(OH)₂ can be prevented under alkaline conditions in the aqueoussolution containing Cd²⁺ ions.

(2) Preparation of Solution B

Selenium powder, in an amount of 0.8612 mmol, and 4.247 mmol of sodiumsulfite (Na₂SO₃) were dissolved in 17 ml of water, and the mixture wasstirred for 2 days at 90° C., thus producing Solution B. The chemicalreactions in Solution B can be conceptually shown by the reaction schemedescribed below.Se+Na₂SO₃→NaHSe+Na₂HSO₃NaHSe→HSe⁻+Na⁺Na₂SO₃ functioned as a reducer in order to ionize Se.

The HSe⁻ ion reacts with oxygen in the air as shown in the following.2HSe⁻+O₂→Se+2OH⁻For this reason, in accordance with increase of pH, the ionconcentration in Solution B is decreased. Therefore, in the presentExample, the surrounding area of Solution B was filled with an inert gasto avoid contact between the air and Solution B.(3) Preparation of a Mixed Liquid of Solution A and a Surfactant

Subsequently, in a conical flask equipped with a magnetic stirrer, 14 mlof Solution A and 4 mmol of a surfactant were mixed, and 10 ml ofmethanol, 10 ml of hydrochloric acid, and 10 ml of water were mixedtherewith. As the surfactant, decyamine (CH₃(CH₂)₉NH₂) was employed.

Decylamine has a specific ratio of the molecular weight of thehydrophilic group (NH₂—) and the molecular weight of the hydrophobicgroup (CH₃(CH₂)₉), that is, hydrophilic group:hydrophobic group=16:141.Therefore, the proportion of the hydrophobic group is high, and for thisreason, decylamine is advantageous to form a reversed micelle in an oilyphase. In addition, decylamine can be easily dissolved in water. Forthis reason, a large amount of decylamine can be dissolved in an aqueousphase. Therefore, in the subsequent step, a large amount of reversedmicelles can be produced in the oily phase.

During mixing of Solution A, decylamine, methanol, and hydrochloricacid, the conical flask was heated on a hot plate. Thereby, thetemperature of the mixed solution was in the range of from 40° C. to 60°C. During mixing, uniformity of the temperature of the mixed solutionwas ensured by stirring the mixed solution with a magnetic stirrer. Bymixing with hydrochloric acid, the pH of the mixed solution wasmaintained at alkaline pH 9 to pH 11. Stirring of the mixed solution wascontinued for 25 minutes.

(4) Mixing with Solution B

Solution B, in an amount of 12 ml, was added to the aforementioned mixedsolution. Thereby, the Cd ions and the Se ions (accurately HSe⁻ ions) inthe system were reacted to form CdSe. The Cd ions were chelated by STNA,and the concentration of the Se ions was low. For this reason, it isbelieved that the reaction rate of the Cd ions and the Se ions wasconsiderably slow. However, growth of CdSe proceeded, and for thisreason, the subsequent step was carried out immediately.

(5) Mixing of an Oily Medium

Immediately after Solution B was added to the aforementioned mixedsolution, 30 ml of toluene was mixed therein as an oily medium, thusproducing a double phase separating liquid in which the continuousaqueous phase directly contacted the continuous oily phase. Furthermore,30 ml of distilled water was added to the aqueous phase. It was observedthat reversed micelles were formed by decylamine in the oily phase inthe double phase separating liquid immediately after toluene was mixed.The temperature of the double phase separating liquid was maintained inthe range of from 40° C. to 60° C.

(6) Separation of the Product

After 25 minutes, the double phase separating liquid in the conicalflask was poured into a separating funnel, and the aqueous phase and theoily phase were separated. The oily phase was centrifuged by means of acentrifuge, and the obtained sedimented product was separated out, thusobtaining 8 mg of yellow-green powder.

Example 2

A powder product was produced by repeating the procedures described inExample 1 under the same conditions as those of Example 1, with theexception of replacing the decylamine of Example 1 with octylamine(CH₃(CH₂)₇NH₂).

Example 3

A powder product was produced by repeating the procedures described inExample 1 under the same conditions as those of Example 1, with theexception of replacing the decylamine of Example 1 with dodecylamine(CH₃(CH₂)₁₁NH₂)

Identification of (CdSe)₃₃ and (CdSe)₃₄ Hollow Polyhedral FineParticles:

(1) Optical Absorption Spectrum Analysis

The product obtained in Example 1, in an amount of 5 mg, was dissolvedin 20 ml of toluene, and the optical absorption spectrum thereof in therange of 2.0 eV to 4.5 eV was obtained. The results are shown in FIG. 1.The bulk CdSe had an absorption peak at 1.7 eV, and the product ofExample 1 had an absorption peak at 3.0 eV. Therefore, it could beconfirmed that the product of Example 1 was a different compound fromthe bulk CdSe. The sharpness of the absorption spectra reveals that thesample is single in atom level.

Mass Analysis

The product obtained in Example 1 was subjected to mass analysis underlinear mode conditions by means of a time of flight method. The measuredsample was prepared by dropping a toluene solution of the product on atarget plate of a spectrometer and drying this. Decylamine of thesurfactant adhering to the surface of the product was evaporated andremoved by irradiation with a nitrogen laser. The results are shown inFIG. 2.

The peak width in FIG. 2 indicates a binary isotope distribution of Cdand Se. A strong peak was observed at n=13, sharp peaks were observed atn=33 and at n=34. In addition, a weak peak was observed at n=19.Therefore, the peaks were respectively identified with (CdSe)₁₃,(CdSe)₃₃, (CdSe)₃₄, and (CdSe)₁₉. FIG. 2 shows that (CdSe)₃₃ and(CdSe)₃₄ were produced in abundance.

(3) Electron Microscope Analysis

The product obtained in Example 1 was subjected to electron microscopeanalysis under conditions of 400 KV employing a transmission model. Theelectron microscope photo is shown in FIG. 3. In FIG. 3, there were somevery small nanoparticles having a particle size of not more than 2 nm,and the lattice fringe observed in bulk crystals could not be observed.Therefore, it could be seen that in Example 1, (CdSe)₃₃ and (CdSe)₃₄particles which were not CdSe bulk crystals were actually produced, andthat the particle size of the aforementioned particles ranged fromapproximately 1.2 to 1.7 nm.

(4) Theoretical Calculation

By carrying out theoretical calculation of the first principle underconditions of the pseudopotential method, the structural stability of(CdSe)₃₃ and (CdSe)₃₄ was calculated. As a result, it was predicted thatthe hollow polyhedral particle structure in the form of a cage shown inFIG. 4 and FIG. 5 having surface structure in which a 4-membered ringand a 6-membered ring were combined would be stable. It was predictedthat, as the number of the molecules of CdSe in which the particles werestable, 13 and the like would be present, in addition to 33 and 34. Infact, in the mass spectral chart shown in FIG. 2, a peak of (CdSe)₁₃ waspresent in addition to peaks of (CdSe)₃₃ and (CdSe)₃₄. Therefore, theaccuracy of the theoretical calculation could be supported. It could beconfirmed from this that (CdSe)₃₃ and (CdSe)₃₄ were hollow polyhedralfine particles. In addition, it was estimated that the maximum particlesize of the (CdSe)₃₄ particle was 1.45 nm. This had good consistencywith the actually observed datum by means of an electron microscope offrom approximately 1.2 to 1.7 nm.

(5) Measurement by X-ray Diffraction

The product obtained in Example 1 was subjected to measurement by X-raydiffraction under the conditions of powders employing a dried sample. Asa source of the X ray, a Cu—Kα was employed. The results are shown inFIG. 6. In FIG. 6, peaks of Wurzite or Zincblend crystals at thevicinity of 2θ=25° appearing in bulk CdSe crystals were not observed.Therefore, it was reconfirmed that the product was not a bulk CdSecrystal.

In FIG. 6, some strong peaks were observed at the region of 2θ=not morethan 20° . The first peak at 3.75° was the base peak for the followingfour high-order peaks. If it is assumed that the first peak is the [100]reflection of a simple cubic structure, the distance between theparticles of the product was calculated as 2.37 nm. This wasconsiderably greater than the size of the (CdSe)₃₃ and (CdSe)₃₄particles. For this reason, it was believed that the surfactant adheringto the surface of the aforementioned particles had some effects thereon.

Therefore, the products of Example 2 and Example 3 employing thesurfactants having different molecular lengths were subjected tomeasurement by X-ray diffraction under the same conditions as describedabove. As a result, in the products of Example 2 and Example 3, thedistances between the particles of (CdSe)₃₃ and (CdSe)₃₄ wererespectively 2.05 nm and 2.72 nm. From the results the changes in thedistance between the particles were derived from the differences inmolecular lengths of the surfactants (octylamine: approximately 1.0 nm;decylamine: approximately 1.2 nm; and dodecylamine: approximately 1.5nm), and it was confirmed that molecules of the surfactant were presentbetween the particles of (CdSe)₃₃ and (CdSe)₃₄ as spacers, as shown inthe conceptual expression below.

Measurement of Photoelectric Effects of the (CdSe)₃₃ and (CdSe)₃₄ HollowPolyhedral Fine Particles:

On a Ti electrode, the (CdSe)₃₃ and (CdSe)₃₄ particles in which thesurfactants were removed were deposited by means of electrophoresis, andtheir photoelectric effects were measured in a selenothiosulphytosodiumsolution under a xenon lamp. On the Pt electrode which was anotherelectrode of the electrode couple, 0.55 V of photoelectric effects wasobserved.

INDUSTRIAL APPLICABILITY

The method for producing hollow polyhedral fine particles of the presentinvention is an organochemical synthetic method in which operations canbe carried out at room temperature. For this reason, the method of thepresent invention can be carried out under mild conditions which aredifferent from those in the case of high-energy consumption typesynthetic methods such as a physical vapor growth method and the like,and can produce hollow polyhedral fine powders at high efficiency inlarge amounts.

The hollow polyhedral fine powders represented by chemical formulae of(CdSe)₃₃ and (CdSe)₃₄ produced by means of the production method of thepresent invention are nanoparticles stable in the atmosphere, and arenovel substances which have not been synthesized heretofore. Asdescribed above, by employing the production method of the presentinvention, hollow polyhedral fine powders which are stable in theatmosphere, other than carbon fullerenes, can be actually synthesized.

In addition, the hollow polyhedral fine powders represented by chemicalformulae of (CdSe)₃₃ and (CdSe)₃₄ produced by means of the productionmethod of the present invention have electronic states which aredifferent from those of bulk crystals of CdSe, and can be employed invarious uses as novel functional materials, and in particular, assemiconductors. In particular, (CdSe)₃₃ and (CdSe)₃₄ can be regularlyarranged, and for this reason, they can be employed as nanomolecularcircuit devices.

1. A method for producing hollow polyhedral fine particles having atomsof a first element and atoms of a second element, wherein atoms of saidfirst element and atoms of said second element are structured to form ashell structure of the hollow polyhedral fine particle in a reversedmicelle composed of a surfactant.
 2. A method for producing hollowpolyhedral fine particles having atoms of a first element and atoms of asecond element, said method comprising the steps of: a first step ofdissolving or dispersing a surfactant, a compound containing atoms ofsaid first element, and a compound containing atoms of said secondelement, in an aqueous medium to obtain an aqueous solution or anaqueous dispersion; a second step of adding an oily medium to saidaqueous solution or dispersion to obtain a double phase contactingliquid in which an aqueous phase and an oily phase directly contact; athird step of forming reversed micelles composed of said surfactant insaid oily phase of said double phase contacting liquid; and a fourthstep of structuring atoms of said first element and atoms of said secondelement in said reversed micelles to obtain hollow polyhedral fineparticles.
 3. The method for producing hollow polyhedral fine particlesaccording to claim 2, further comprising a fifth step of separating andrecovering said hollow polyhedral fine particles from said oily phase,after said fourth step.
 4. The method for producing hollow polyhedralfine particles according to claim 2, wherein said first element and saidsecond element are the same element.
 5. The method for producing hollowpolyhedral fine particles according to claim 2, wherein said firstelement and said second element are different elements.
 6. The methodfor producing hollow polyhedral fine particles according to claim 5,wherein said first element is Cd, and said second element is Se.
 7. Ahollow polyhedral fine particle represented by the following chemicalformula: (CdSe)₃₃ or (CdSe)₃₄.
 8. The method for producing hollowpolyhedral fine particles according to claim 1, wherein said firstelement and said second element are the same element.
 9. The method forproducing hollow polyhedral fine particles according to claim 1, whereinsaid first element and said second element are different elements. 10.The method for producing hollow polyhedral fine particles according toclaim 9, wherein said first element is Cd, and said second element isSe.
 11. A hollow polyhedral fine particle comprising: atoms of a firstelement, and atoms of a second element, wherein atoms of said firstelement and atoms of said second element are structured to form a shellstructure of the hollow polyhedral fine particle in a reversed micellecomposed of a surfactant.
 12. A hollow polyhedral fine particleaccording to claim 11, wherein said first element and said secondelement are the same element.
 13. A hollow polyhedral fine particleaccording to claim 11, wherein said first element and said secondelement are different elements.
 14. A hollow polyhedral fine particleaccording to claim 13, wherein said first element is Cd, and said secondelement is Se.
 15. A hollow polyhedral fine particle according to claim13, wherein said first element is selected from the group of Group II toGroup VI elements.
 16. A hollow polyhedral fine particle according toclaim 15, wherein said first element is selected from the group of GroupII elements.
 17. A hollow polyhedral fine particle according to claim16, wherein said first element is selected from the group of Cd and Zn.18. A hollow polyhedral fine particle according to claim 15, whereinsaid second element is selected from the group of Group II to Group IIto Group VI elements.
 19. A hollow polyhedral fine particle according toclaim 18, wherein said second element is selected from the group ofGroup VI elements.
 20. A hollow polyhedral fine particle according toclaim 19, wherein said second element is selected from the group of S,Se, and Te.